Crankpin

It has a cylindrical surface, to allow the crankpin to rotate relative to the "big end" of the connecting rod.

This usually requires an offset between the cylinders in each bank, resulting in a simple connecting rod design.

If a cylinder offset is not used, then the connecting rods must be articulated or forked at the big end.

Cylindrical crank pins were fitted onto the driving wheels of steam locomotives.

The crank pin was usually made of high-quality steel because it had to withstand high forces.